Video scrambling techniques are known for use in home cable TV systems where it is desired to restrict programs to authorized viewer-subscribers, and also for use in military applications such as transmission of video signals from an airplane. In between these extremes are industrial applications such as video conferencing, videophone, two-way TV, etc.
The difficulty in encryption of a video signal is that the signal is analog, has a very high information rate, and re-ordering of analog video information in a line may adversely affect the signal and noise spectra seen at a receiver. With the advent of VLSI electronics technology which become feasible only in the last few years, very quick digital storage methods became available using fast RAMs that can store and retrieve video information in real time in a frame buffer. This led to the possibility of scrambling the video signals by digital methods, leaving the transmission format intact.
Examples of these digital methods include line reversal, line position modulation, line segment swapping, and line dispersal, all of which are described in a paper entitled "High Security Television Transmission Using Digital Processing", by R. Kupnicki et al., IEEE publication 84CH2069-3 (1984). This paper calls attention to the need to select an appropriate scrambling algorithm in order not to increase the bandwidth of the original video signal, so that a standard video receiver can be employed. Another similar technique involves line rotation and is described in U.S. Pat. No. 4,070,693 to Shutterly.
When these scrambling techniques are examined from the aspect of cryptographic strength, it is seen that correlation techniques make it possible to decode the schemes by comparing and matching lines or segments until the original arrangement of these lines or segments is found in order to recover a significant portion of the entire picture.
An additional digital method of scrambling a video signal calls for random permutation of the pixels in a line, as described in British patent No. 1,590,579 to Taylor. Again, it is stated that the nature of a video scrambler is to destroy the relationship between adjacent picture points, resulting in a requirement for a higher transmission bandwidth for the video signal. Otherwise, distortion of the picture makes the technique ineffective.
One very common video scrambling technique is provided by a system known by the trademark videoCipher, as made available by M/A-COM Linkabit, Inc., and described in a paper entitled "The VideoCipher Scrambling System" by P. Mulroney et al., IEEE publication 84CH2069-3 (1984). This system is also described in European patent application number 84303322.6, and is based on permutations of portions of video information lines. The number of possible permutations is very high, but since the line portions are long, the number of jumps within the video signal is kept low and so the bandwidth increase is insignificant.
However, the long line portions make for easier correlation attacks on the scrambling algorithm by comparing and matching line portions.
Therefore, it would be desireable to provide a video scrambling technique which has a higher cryptographic strength and operates within the standard video signal bandwidth.